Electron field emission is an emission of electrons induced by external electromagnetic fields. Field emission can happen from solid and liquid surfaces, or individual atoms into vacuum or open air, or result in promotion of electrons from the valence to conduction band of semiconductors.
Field emission in pure metals occurs in high electric fields: the gradients are typically higher than 1000 volts per micron and strongly dependent upon the work function of the metal. Electron sources based on field emission have a number of applications, including electron sources for high-resolution electron microscopes and electron-beam lithography. Field emission is explained by quantum tunneling using the Fowler-Nordheim equations.
Although electron field emitters have been known for some time and there are a number of methods that have been employed to implement these emitters, most emitters have an aspect ratio, that is a height-to-diameter ratio, that is very limited. It is often desired that the emitter have an atomically-sharp-shaped end, be composed of highly conductive materials, and have a height from the substrate surface of a significant amount. Consequently, structures meeting these requirements are difficult to implement.
More recently, enormous interest has been shown in using carbon nanotubes, either single-walled or multi-walled carbon nanotubes, as field emitters due to the excellent material properties of carbon nanotubes. In general, carbon nanotubes have a limited height that they can be grown and this height is not sufficient to reduce the electrical field effects from the substrate floor. Therefore, it is desired that the carbon nanotube be implemented onto a highly-conductive material formed into high-aspect ratio structure. These structures are also difficult to implement.